The objective of this application is to develop a computational model to study mechanisms of mitochondrial toxicity of the nucleoside analogs used in HIV therapy. Highly active antiretroviral therapy (HAART) typically contains two or more nucleoside reverse transcriptase inhibitors (NRTIs). The NRTIs are analogs to the four natural nucleosides and follow the same cellular metabolic pathways as their natural nucleoside counterparts, up to incorporation of the tri-phosphate form of the NRTIs into replicating DNA strands. When an NRTI molecule is added to the replicating DNA strand replication is terminated. The NRTIs' antiviral activity occurs through this interference with viral DNA replication. Unfortunately, NRTIs can also interfere with the replication of mitochondrial DNA (mtDNA). This is suspected to be the primary mechanism behind the mitochondrial toxicity that is a serious problem with HAART. This toxicity can appear in many forms, from lactic acidosis, which is often lethal, to cardiomyopathy, peripheral neuropathies and hepatotoxicity. In this exploratory project we propose to develop a computational model of the metabolic pathways within mitochondria of both the four natural nucleosides and of two nucleoside analogs, AZT and 3TC. The computational model includes the transport of deoxynucleosides and deoxynucleotides between the mitochondrial matrix and the cell cytoplasm, the phosphorylation of the nucleosides and the polymerization of mtDNA. The results of these simulations will be compared with experiments conducted by our collaborator, and will be used to interpret these experiments. [unreadable] [unreadable] [unreadable]